Increased density of glucagon receptors in liver from endurance-trained rats

The binding properties of glucagon receptors were determined in plasma membranes isolated from liver of untrained (n = 6) and swimming endurance-trained Sprague-Dawley male rats (n = 7; 3 h/day, 5 days/wk, for 8 wk). Plasma membranes were purified from liver by aqueous two-phase affinity partitioning, and saturation kinetics were obtained by incubation of plasma membranes (10 microg of proteins/150 microl) with (125)I-labeled glucagon at concentrations ranging from 0.15 to 3.0 nM for 30 min at 30 degrees C. Saturating curve analysis indicated no difference in the affinity of glucagon receptors (0.57 +/- 0.06 and 0.77 +/- 0.09 nM in untrained and trained groups, respectively) but a significant higher glucagon receptor density in liver from untrained vs. trained rats (3.09 +/- 0.12 vs. 4.28 +/- 0.19 pmol/mg proteins). These results suggest that the reported increase in liver glucagon sensitivity in endurance-trained subjects (Drouin R, Lavoie C, Bourque J, Ducros F, Poisson D, and Chiasson J-L. Am J Physiol Endocrinol Metab 274: E23-E28, 1998) could be partly due to an increased glucagon receptor density in response to training.